1. Field of the Invention
The present invention relates to a six-stroke engine including a cylinder injector that directly injects fuel into a combustion chamber.
2. Description of the Related Art
As a conventional technique of improving the fuel efficiency of an engine, there exists a so-called stratified charge combustion technique of concentrating a small amount of fuel in the vicinity of a spark plug and burning it. A conventional engine capable of performing stratified charge combustion is described in, for example, Japanese Patent Laid-Open No. 2001-342836.
To concentrate a small amount of fuel in the vicinity of the spark plug, the engine disclosed in Japanese Patent Laid-Open No. 2001-342836 uses an arrangement to generate a tumble that is a swirl flow of intake air in the cylinder. The tumble occurs when the intake air that has flowed from the intake port into the cylinder flows in the axial direction of the cylinder and reverses at the top of the piston. The intake port and the piston top of the engine described in Japanese Patent Laid-Open No. 2001-342836 are made in special shapes so as to keep the tumble even at the latter half of the compression stroke.
The intake port is arranged such that the tilt angle with respect to the axis of the cylinder becomes smaller than normal. A spherical concave portion is provided at the piston top. In this engine, the fuel is directly injected into the combustion chamber by a cylinder injector during the compression stroke. The fuel hits the intake air flowing in the tumble and stays in the vicinity of the spark plug.
As an engine capable of improving the fuel efficiency, a six-stroke engine is known. The six-stroke engine operates in one cycle of six strokes: four strokes including an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke and an additional compression stroke and expansion stroke. A conventional six-stroke engine is described in, for example, Japanese Patent Laid-Open No. 2004-116305.
The six-stroke engine disclosed in Japanese Patent Laid-Open No. 2004-116305 is operated by executing six strokes including an intake stroke, a compression stroke without ignition, an expansion stroke without combustion, a compression stroke with ignition, an expansion stroke with combustion, and an exhaust stroke in this order. In this six-stroke engine, fuel is supplied into the cylinder or intake channel in the intake stroke. In the subsequent compression stroke without ignition and expansion stroke without combustion, the fuel and intake air are stirred and mixed in the cylinder.
The intake port in Japanese Patent Laid-Open No. 2001-342836 is arranged to greatly change the direction of intake air flow to cause a tumble. In the engine described in Japanese Patent Laid-Open No. 2001-342836, the intake port, the piston top, and the fuel (spray) injected from the cylinder injector need to be made in optimum shapes. For this reason, the intake air volume is small in the engine described in Japanese Patent Laid-Open No. 2001-342836, and there is a limit on increasing the output.
The six-stroke engine described in Japanese Patent Laid-Open No. 2004-116305 cannot perform stratified charge combustion, and it is therefore difficult to further improve the fuel efficiency.